1. Field of the Invention
The present invention relates to the field of input choppers for such uses as in operational amplifiers, instrumentation amplifiers and analog-to-digital converters.
2. Prior Art
Instrumentation amplifiers and analog-to-digital converters are frequently used for sensor interfacing. In current-sense applications for power management of cell phones, laptops, and electric cars, a small differential-mode voltage in the order of millivolts has to be measured across a current-sense resistor in the presence of a large common-mode voltage on the sense resistor on the order of Volts or tens of Volts higher than the supply voltage. To solve this differential-mode voltage measurement problem while the common-mode voltage is beyond the supply-rail, often a part of the input circuitry (which may be a resistor bridge or a voltage-to-current converter) is allowed to draw its common-mode supply current from the sense resistor. This input common-mode supply current often has an undesired influence on the system to be measured. Therefore, instrumentation amplifiers and analog-to-digital converters with capacitive-coupled input choppers have been developed that do not draw input common-mode supply current, while being able to sense beyond their supply-rail voltage. One such prior art technique is to use an input chopper followed by a capacitive coupled differential input. A second chopper rectifies the square wave back into the input waveform.
Another prior art technique is to use transformers, as shown in U.S. Pat. No. 7,714,757. That solution is expensive to implement, both in cost and in circuit board area, as it cannot easily be fully integrated.